This invention relates to radiation detectors and, more particularly, to novel and highly-effective radiation detectors made of a semiconductor such as hyperpure germanium and having a coaxial configuration.
The use of semiconductors to detect incident radiation is a well-developed art. When a semiconductor diode is subjected to a reverse bias, the ionization caused by incident radiation permits a pulsed current to flow. The current can be detected and analyzed to provide a great deal of information about the incident radiation.
To minimize noise and maximize sensitivity, it is necessary to minimize leakage currents. Surface leakage currents are a major problem, and a means for reducing them in a planar-type lithium-drifted silicon detector is disclosed by J. Llacer in "Geometric Control of Surface Leakage Current and Noise in Lithium Drifted Silicon Radiation Detectors", IEEE Trans. on Nucl. Sci. NS-13 No. 1, 93 (1966). Basically, Llacer proposed an "inverted T" geometry including a groove between the intrinsic and p regions of a lithium-drifted planar silicon detector, with structural and functional results as indicated in FIGS. 8, 9 and 12 of his paper.
However, detectors of this type are not as advantageous in many applications as are coaxial detectors. Coaxial detectors have generally the shape of a hollow cylinder with outer and inner cylindrical surfaces. One of the cylindrical surfaces--generally the outer one--has a cylindrical n.sup.+ contact and the other a cylindrical p.sup.+ contact. The cylinders formed by the contacts are opened at at least one and sometimes at both ends.
Coaxial detectors are preferable to planar detectors in applications such as the detection of high-energy (e.g., 1 MeV) gamma radiation in that coaxial detectors can be made much larger than planar detectors and perform admirably in large sizes. Planar detectors cannot be readily made in large sizes, and near the upper limit of their size range they exhibit non-uniform electric field characteristics that degrade performance. A particularly desirable type of coaxial detector would be one having a thin contact on the outer surface, since a thin contact absorbs little of the energy of incident radiation prior to penetration of the radiation into the sensitive volume of the detector. However, this type of coaxial detector has been unsatisfactory because of excessive surface-leakage currents even at voltages far below those necessary for depletion. Current in the form of minority carriers is especially prone to leak from the open end of a thin p.sup.+ contact.
It is also particularly desirable to choose as the semiconductor a material the electrical characteristics of which remain stable at room temperature. Such materials include hyperpure germanium: i.e., germanium in which the concentration of net residual active impurities is below 5.times.10.sup.10 cm.sup.-3. Radiation detectors are operated at cryogenic temperatures, but hyperpure germanium detectors have great stability and need not be shipped and stored at cryogenic temperatures. Lithium-drifted germanium detectors, on the other hand, must be not only operated but also shipped and stored at cryogenic temperatures. In case of accidental loss (as by evaporation) of the liquid nitrogen bath used as a refrigerant, a lithium-drifted germanium detector is ruined by heat in just a few hours. Lithium-drifted silicon detectors under the same conditions are serviceable for only a matter of months.